Now I'm using mod_cluster, I cannot get this to work. When hitting the URL http://cluster.domain.com/ I just get a 404 from my application. Logging from mod_rewrite says that the rule has matched, but it doesn't make its way through to JBoss.

BTW - if I enter http://cluster.domain.com/index.html then my app picks this up fine.

you want / or /index.html to be mapped to your application in JBoss so it must be the ROOT webapp (server/default/deploy/ROOT.war), once it is working in JBoss directly you should remove the Rewrite directives.